Electrophotographic process utilizing friction charging

ABSTRACT

A method for electrostatically charging the surface of a photoconductive insulative layer which is formed by dispersing an inorganic n-type photoconductive powder in a binder, which is primarily a vinyl chloride-vinyl acetate copolymer, which comprises rubbing the insulative layer with a friction material so as to produce a negative charge on the surface of the insulative layer. Representative photoconductors, such as ZnO, and friction materials, such as a cotton velvet cloth, are set out in the specification. A modification of the process entails producing the friction by rubbing the insulative layer while a highly insulative liquid is maintained on its surface.

United States Patent Inventors Masamichi Sato;

Satoru Honio; Masaaki Takimoto, Saitama, Japan Appl. No. 752,472 Filed Aug. 14, 1968 Patented Mar. 9, 1971 Assignee Fuji Shashiu Film Kabushiki Kaisha Ashigara-Kamigun, Kanajawa, Japan Priority Aug. 15, 1967 Japan 42/52318 ELECTROPHOTOGRAPHIC PROCESS UTILIZING FRICTION CHARGING [56] References Cited UNITED STATES PATENTS 2,297,691 10/1942 Carlson 96/1.4 3,052,539 9/1962 Greig 96/1.8 3,121,006 2/1964 Middleton 96/1.5 2,987,660 6/1961 Walkup 317/262 3,172,024 3/1965 Gundlach 317/262(A) Primary Examiner-Lee T. Hiri Assistant Examiner-C. L. Yates AttorneySughrue, Rothwell, Mion, Zinn and M acpeak ABSTRACT: A method for electro'statically charging the surface of a photoconductive insulative layer which is formed by dispersing an inorganic n-type photoconductive powder in a binder, which is primarily a vinyl chloride-vinyl acetate ELECTROPHOTOGHIC PROCESS UTILIZING FMQTION CHGING BACKGROUND OF Tl-IE INVENTION 1 Field of the Invention The present invention relates to an electrophotographic method for charging a photoconductive insulative layer utilizing friction.

2. Description of the Prior Art As electrophotographic processes generally make use of an electrostatic latent image, proving a photoconductive insulative layer with a blank electrostatic charge is generally an important part of such a process. I

For example, a corona discharge is commonly applied, and a charging method utilizing a simple mechanical device is also available, i.e., the surface of a recording layer is rubbed with an appropriate material to achieve triboelectric charging. Such a method is described in US. Pat. No. 2,297,691.

SUMMARY OF THE INVENTION chloride-vinyl It has been found that improved results may be obtained when a photoconductive insulative layer, comprising an ntype n-type photoconductive powder, such as ZnO, which is dispersed in a vinyl chloride vinyl acetate copolymer'with a composition ratio of vinyl chloride to vinyl acetate of from 95:5 to 55:45 parts by weight, is rubbed with a friction material, such as a cotton velvet cloth, to negatively charge the layer.

The invention utilizes the differences in the triboelectric properties of the materials involved.

In one particular modification, the friction charging is performed while the surface of the insulative layer is maintained in contact with a highly insulative liquid, such as an aliphatic hydrocarbon.

The invention thus has as its object the provision of a process which will yield an improved image of extremely high quality when developedDESCRIPTlON OF THE PREFERRED EMBODIMENTS The present inventors have found that if a light sensitive layer, comprising an n-type photoconductive semiconductor, such as photoconductive zinc oxide, titanium dioxide, ect., dispersed in a binder comprising a vinyl chloride-vinyl acetate copolymer as its main constituent, is rubbed with a friction material which is positive in the triboelectric series with respect to said layer, a copy which is superior in quality to other combinations can be obtained.

Suitable prior art insulative binders for use with zinc oxide, are poiysiloxane resins alkyd resins, epoxy esters, polyacrylic esters, polymethacrylic esters, styrene containing polymers, and the like. With respect to triboelectric characteristics, all of these resins are relatively positive, and powdery zinc oxide is extremely positive. Extremely positive "as used above, means that using ethylcellulose (ethoxized percentage: 47.5- 49.0 percent) as an empirically typical positively chargeable resin, zinc oxide is very positive in comparison. Nitrocellulose can be selected as a typical negatively chargeable resin.

Accordingly, if a light sensitive layer composed of any of the above described typical positively chargeable resins and ZnO is rubbed with an appropriate friction me material, it shouldbe positively charged. However, a satisfactory visible image cannot be obtained upon development when these prior art resins are used. It is believed that the reason for this is that the Zn() acts as an n-type semiconductor layer and even if it is charged the electric charges immediately disappear.

An image of high density is also hard to obtain even if an insulative material is applied as a supporting material or provides a relatively thick insulative layer between a conductive supporting material and a sensitive layer, or even where (for the purpose of minimizing the effect of leakage) an image-exposure is first applied to a light sensitive layer and its surface is then rubbed with a friction material in a liquid developer comprising an insulative liquid as carrier liquid.

As negatively chargeable resins, vinyl chloride copolymers, nitrocellulose, vinylidene chloride copolymers andthe like are available. As a result of forming light sensitive layers by dispersing ZnO in these resins, it was ascertained that the latter two have a low charge acceptability and are difficult to process stably. On the other hand, it was experimentally confirmed that if a photoconductive insulative layer composed of a vinyl chloride-vinyl acetate copolymer and zoo is rubbed with a friction material which is strongly positive in the triboelectric series in comparison therewith, an image of extremely high quality can be obtained, regardless of whether such charging process is carried out prior to or after exposure.

The image quality obtained by using this combination compared quite favorably with the image quality one obtained with a normal corona charging process. Such satisfactory results could be obtained only in the case of using a combination of vinyl chloride-vinyl acetate copolymer and 2:10 or another n-type photoconductive material.

The binder used may contain a small quantity of plasticizer, but such a component is liable to stain the surface of a friction material when used repeatedly, and therefore it is preferably preferable that a plasticize not be used. With respect to other ingredients which may be present, polymers of nonmigrating characteristics are desirable, for instance, vegetable oil modified alkyd resins, epoxy ester resins, vinyl monomer modified alkyd resin, and thelike, are available.

As the above ingredients are charged in a strongly positive manner, they should not be added in excessive quantity. A composition of 60 parts by weight of vinyl chloride-vinyl acetate copolymer and 40 parts of other components may be regarded as being the upper limit of binder composition. If a liquid development is utilized, it is desirable to cure the aforesaid alkyd resins, epoxy ester resins and the like by means of curing agent prior to development so as to improve the charge decay characteristics in the liquid.

With respect to the vinyl chloride vinyl acetate copolymer, one containing more than 5 percent and less than 45 percent (by weight) vinyl acetate is suitable. It is difficult to dissolve polyvinyl chloride homopolymer in many solvents, while a content of more than 45 percent vinyl acetate lowers the insulating properties and the advantageous friction charge characteristic of polyvinyl chloride. These elements should be kept in mind when formulating the copolymer of the present invention.

As a supporting material, generally one possessing good conductivity may be used, but an insulative supporting material can be utilized when the friction step occurs after exposure.

As representative n-type photoconductive materials useful in the present invention there are titanium dioxide, zinc oxide, cadmium sulfide, cadmium sulfide-cadmium carbonate mixtures, cadmium sulfide-zinc sulfide mialed crystals, and the like. Mixtures of these substances may also be used.

As friction charging materials for the light sensitive material, the most positive in comparison to a photoconductive layer in the triboelectric series are preferred Of course, proper mechanical properties are preferable. For instance, cotton velvet cloth, rayon, wool, tannedleathcr, nylon cloth, polyurethane sponge, and the like are especially desirable.

By this method, a high surface potential can be attained; therefore, the used a developing process ensuring especially high sensitivity is not always an absolute requirement in obtaining an acceptable image, that is, an image of high density can be obtained by means of cascade development, magnetic brush development, and the like. An image of higher quality can be obtained by liquid development. If a nonpolar liquid hydrocarbon or a liquid fluorinated hydrocarbon is applied as the carrier liquid, the surface charge will be maintained for a long period of time, even in the liquid, thus making it possible to obtain an excellent image which exhibits very little "edge effect." Such improved results may be attributed to the specific use of the vinyl chloride vinyl acetate copolymer.

As stated above, an image with high resolution can be obtained by means of a liquid development, but this type of developing process is liable to be accompanied by fog in the image element background. To obviate this fog" problem the following is a specially desirable modification of the present invention. Specifically, the friction generated between the friction material and the light sensitive layer is caused to take place under an insulative liquid. This process may be carried out either prior or subsequent to image exposure. If this method is followed, the fog density will be remarkably reduced. it is theorized that the mechanism of this aspect of the invention can be explained as of follows: when a thin layer of insulative liquid is formed on the surface of the light sensitive layer, the adhesion of toners to the surface of the light sensitive layer due to forces other than electrostatic forces (i.e., adsorptive attraction, trapping" into surface irregularities, etc.) in the subsequent developing process will be remarkably reduced. In addition, when the charge acceptance of the light sensitive layer in the insulative liquid is higher than the carrier liquid of the developer, the lifetime" of a latent image will be extended, and the image 'density will be increased due to the presence of such a thin liquid layer. In general, the liquid initially applied is a purified, highly insulative nonpolar liquid, and since the developer carried liquid contains resin and other components, the above requirement is satisfied.

The liquid which is applied at the time of the generation of the friction should be such one that can be mixed with a developer which is applied subsequently and which will not attack a light sensitive layer. Typical examples are aliphatic hydrocarbons, alicyclic hydrocarbons and fluorinated hydrocarbons.

As is clear from the above description, the present invention makes an extremely simple electrography possible. in addition, the process may be practical in a compact copying machine and therefore it has a high value from an industrial viewpoint.

The process of this invention (herein friction charging is employed) is more profitably used in an electrophotographic process in which the photoconductive layer is charged after the image-exposure, than in the process wherein a corona discharge is employed. This is due to the fact that when the photoconductive layer is image-exposed prior to charging and is then charged by a corona discharge utilizing a photoconductive after effect, sharp images will not usually be obtained. This result may be explained as follows: at the boundary of an exposed and an unexposed area, the conductivity of the former remains high, while the latter is low, so that when both of these ares areas are subjected to a corona discharge, the charge is barely accumulated in the former area, while the potential of the latter area absorbs the charge rapidly. If the corona discharge is continued, this will cause the preferential deposition of corona-produced ions onto the exposed area, which will be repelled by the adjacent charged area which has the same polarity as the ions. Accordingly, the exposed areas which are adjacent the unexposed areas receive a higher concentration of con corona-produced ions and recover from light fatigue more rapidly than other exposed areas. Since such a phenomenon results immediately after the beginning of the charging by the corona, an image with an indefinite edge results.

in opposition to the above-described result, according to the process of the present invention, when friction charging is employed after image-exposure, an image without the abovementioned drawbacks will be obtained.

The invention will now be explained in greater detail by the following examples.

EXAMPLE 1 A light sensitive layer comprising 100 parts of photoconductive ZnO powder and 25 parts of vinyl chloride-vinyl acetate copolymer (containing 35 percent by weight of vinyl acetate) was coated to a thickness of 5 [LOB a sheet of paper which has had been rendered conductive. After the dark adaptation of the layer, an image exposure was applied by means of a contact exposure. The light sensitive paper was then clipped into a liquid developer comprising a paste of a kneaded mixture of phthalocyanine blue pigment and rosin-modified phenol-formaldehyde resin dispersed in cyclohexane. The surface of the light sensitive layer was then lightly rubbed several times with a polyurethane sponge. The pigment maintained a positive charge in the liquid, while the light sensitive layer which was rubbed with polyurethane was negatively charged. Thus, an attractive" development could be achieved. The resultant image was quite sharp, and showed no indistinct outline along the circumference of image as would be observed in an ordinary image involving the Kalmann method of electrophotography. The image density was also excellent.

In another modification of the present invention, rubbing (to generate friction) was carried out in kerosene which did not contain any toner particles, and subsequently the light sensitive chloride-vinyl into the above-described developer. A lower fog density than that obtained in the above-mentioned example was obtained.

EXAMPLE 2 A light sensitive layer consisting of 50 parts of photocom ductive titanium dioxide, 50 parts of photoconductive zinc oxide, 27.5 parts of a vinyl chloride-vinyl acetate copolymer (the same type as used in example I) 2.5 parts of the epoxy ester of dehydrated castor oil fatty acid (oil length 40 percent and 10 parts per 1000 parts of rosebengal, was coated to a thickness of 5 p. on a sheet of the same type base paper as used in example 1.

While under dark adaptation conditioning, the surface of the light sensitive layer was rubbed several times with a piece of cotton velvet cloth and an image exposure was applied by the projection method. A positive line image was chosen as the original. The electrostatic latent image was developed by a cascade developer containing positivelycharged toners, and a positive reproduction of the original was obtained.

In addition, an excellent image was also achieved by using a liquid development.

As an improved modification of the present invention, cotton velvet cloth impregnated with purified kerosene was used to rub the surface of the light sensitive layer in the same manner as described above. After subsequent exposure and development, a homogeneous image with a low fog density was obtained.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

lclaim:

1. An electrophotographic method for electrostatically charging the surface of a photoconductive insulative layer formed by dispersing an inorganic n-type photoconductive powder in a binder comprising as its main constituent a vinyl chloride-vinyl acetate copolymer with a composition ratio of vinyl chloride to vinyl acetate ranging from about :5 to about 55:45 parts by weight which comprises rubbing said insulative layer with a friction material so as to produce a negative charge on the surface of said insulative layer by friction while maintaining a highly insulative liquid on the surface of said insulative layer so that the generated friction is caused to take place within the insulative liquid.

2. The electrophotographic method of claim 1 wherein said n-type photoconductive material is selected from the group consisting of titanium dioxide, zinc oxide, cadmium sulfide, a mixture of cadmium sulfide and cadmium carbonate, and a mixed crystal of cadmium sulfide and zinc sulfide.

3. The electrophotographic method of claim 1 wherein said friction material is selected from the group son consisting of cotton velvet, rayon, wool, a tanned leather, nylon, and polyurethane.

4. The electrophotographic method of claim l wherein said surface of a photoconductive insulating layer is subjected to a liquid-developing agent to be employed in the subsequent developing process is one to be mixed with the liquid which is applied at the time of the generation of the friction. 

2. The electrophotographic method of claim 1 wherein said n-type photoconductive material is selected from the group consisting of titanium dioxide, zinc oxide, cadmium sulfide, a mixture of cadmium sulfide and cadmium carbonate, and a mixed crystal of cadmium sulfide and zinc sulfide.
 3. The electrophotographic method of claim 1 wherein said friction material is selected from the group son consisting of cotton velvet, rayon, wool, a tanned leather, nylon, and polyurethane.
 4. The electrophotographic method of claim 1 wherein said surface of a photoconductive insulating layer is subjected to a frictional charging in the liquid after the image-exposure carried out in a substantially noncharged and dark-adapted condition of the layer.
 5. The electrophotographic method of claim 1 wherein the liquid-developing agent to be employed in the subsequent developing process is one to be mixed with the liquid which is applied at the time of the generation of the friction. 